Many inverter circuits have previously been used including those with pulse width modulation to establish variable output power. The prior art has known circuits with feedback from the output of the inverter to control the input drive signal in accordance with some electrical output condition such as voltage or current. The prior art has known a drive transformer used with two alternately conducting semiconductors so that when the drive transformer was energized of one polarity a first semiconductor conducted and when energized in the opposite polarity a second semiconductor in the inverter conducted. The difficulty with such a unit was that the drive transformer had a core material exhibiting a rectangular hysteresis loop and this core material was capable of being saturated. A variable amount of saturation in accordance with variable conduction times, for pulse width modulation, meant that the operation of the circuit was erratic, especially under rapidly changing conditions, being dependent upon the amount of conduction time in the previous cycle because of the variable degree of saturation.
The prior art inverter circuits have also included those of relatively high current carrying capability, but where this has been attempted to be combined with high frequency of operation, the semiconductors used, for example transistors, have been of the type which exhibited a relatively high forward voltage drop yet a relatively low reverse blocking voltage. This has made the operation of such an inverter circuit subject to potential failure if the control voltage on a semiconductor should be too high in the reverse direction. Also such inverter circuits have often been ones wherein it was difficult to achieve a turn-off of a particular semiconductor at the proper time in order to control the width of the pulses in the pulse width modulated inverter. Still further in such prior art inverters, especially during transient conditions, a suddenly applied load might tend to cause the inverter to have a greatly increased output which could overload the current carrying capabilities of the inverter semiconductors.